1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to an array substrate for a transflective liquid crystal display device.
2. Discussion of the Related Art
In general, the liquid crystal display (LCD) device includes two substrates, which are spaced apart and facing each other, and a liquid crystal layer interposed between the two substrates. Each of the substrates includes an electrode and the electrodes of each substrate are also facing each other. Voltage is applied to each electrode and an electric field is induced between the electrodes. An alignment of the liquid crystal molecules is changed by the varying intensity or direction of the electric field. The LCD device displays a picture by varying transmissivity of the light varying according to the arrangement of the liquid crystal molecules.
Because the liquid crystal display (LCD) device is not luminescent, it needs an additional light source in order to display images. The liquid crystal display device is categorized into a transmissive type and a reflective type depending on the kind of light source.
In the transmissive type, a back light behind a liquid crystal panel is used as a light source. Light incident from the back light penetrates the liquid crystal panel, and the amount of the transmitted light is controlled depending on the alignment of the liquid crystal molecules. Here, the substrates must be transparent and the electrodes of each substrate must also be formed of transparent conductive material. As the transmissive liquid crystal display (LCD) device uses the back light as a light source, it can display a bright image in dark surroundings. By the way, because an amount of the transmitted light is very small for the light incident from the back light, the brightness of the back light must be increased in order to increase the brightness of the LCD device. Consequently, the transmissive liquid crystal display (LCD) device has high power consumption due to the back light.
On the other hand, in the reflective type LCD device, sunlight or artificial light is used as a light source of the LCD device. The light incident from the outside is reflected at a reflective plate of the LCD device according to the arrangement of the liquid crystal molecules. Since there is no back light, the reflective type LCD device has much lower power consumption than the transmissive type LCD device. However, the reflective type LCD device cannot be used in dark places because it is depends on an external light source.
Therefore, a transflective LCD device, which can be used both in a transmissive mode and in a reflective mode, has been recently proposed. A conventional transflective LCD device will be described hereinafter more in detail.
FIG. 1 is an exploded perspective view illustrating a conventional transflective LCD device. The conventional transflective LCD device 11 has upper and lower substrates 15 and 1, which are spaced apart from and facing each other, and also has liquid crystal 23 interposed between the upper substrate 15 and the lower substrate 1. The liquid crystal 23 has positive or negative dielectric anisotropy.
A gate line 4 and a data line 16 are formed on the inner surface of the lower substrate 1. The gate line 4 and the date line 16 cross each other to define a pixel area “P”. The pixel area “P” includes a transmissive region “A” and a reflective region “B”. A thin film transistor “T” is situated at the crossing of the gate line 4 and the data line 16. A pixel electrode 30, which is connected to the thin film transistor “T”, is formed in the pixel area “P”.
Meanwhile, a black matrix 19, which has an opening corresponding to the pixel electrode 30, is formed on the inside of the upper substrate 15, and a color filter 17 corresponding to the opening of the black matrix 19 is formed on the black matrix 19. The color filter 17 is composed of three colors: red, green and blue. Each color corresponds to a pixel electrode 30. Subsequently, a transparent common electrode 13 is formed on the color filter 17.
In the conventional transflective LCD device of FIG. 1, when a voltage is applied to the pixel electrode 30 and the common electrode 13, an electric field is induced between the pixel electrode 30 and the common electrode 13 in a direction perpendicular to the upper and lower substrates 15 and 1. Molecules of the liquid crystal 23 are arranged by the electric field and light is emitted through the arranged liquid crystal 23 from a back light (not shown) disposed below the conventional transflective LCD device, so that images are displayed.
FIGS. 2 and 3 show an array substrate for a conventional liquid crystal display (LCD) device. FIG. 2 is a plan view of the array substrate for a conventional (LCD) device, and FIG. 3 is a cross-sectional view along the line III—III of FIG. 2.
In FIG. 2 and FIG. 3, a gate electrode 2 and a gate line 4 are formed on a substrate 1. The gate line 4 extends horizontally in the context of the figure and the gate electrode 2 is connected to the gate line 4. A gate insulator 6 covers the gate electrode 2 and the gate line 4, and an active layer 8 is formed on the gate insulator 6. An ohmic contact layer 10 of doped amorphous silicon is formed on the active layer 8.
Next, a source electrode 12 and a drain electrode 14 are formed on the ohmic contact layer 10. The source electrode 12 is connected to a data line 16, which extends vertically in the context of the figure and crosses the gate line 4 to define a pixel region “P”. The ohmic contact layer 10 lowers contact resistance between the active layer 8 and the two electrodes 12 and 14. A capacitor electrode 17, which is made of the same material as the data line 16, is also formed on the gate insulator 6. The capacitor electrode 17 overlaps the gate line 4 to form a storage capacitor. A thin film transistor “T” includes the gate electrode 2, the source electrode 12 and the drain electrode 14. The active layer 8 exposed between the source electrode 12 and the drain electrode 14 becomes a channel “CH” of the thin film transistor “T” when carriers flow between the source electrode 12 and the drain electrode 14.
A first passivation layer 18 covers the source electrode 12, the drain electrode 14, the data line 16, and the capacitor electrode 17. The first passivation layer 18 has a first transmissive hole 20, which exposes a part of the substrate 1 through the gate insulator 6. The first transmissive hole 20 is to equalize the brightness of a transmissive mode with the brightness of a reflective mode. At this time, it is good that the first passivation layer 18 is made of a benzocyclobutene (BCB) or an acrylic resin. Next, a reflector 26 is formed on the first passivation layer 18. The reflector 26 has a first opening 26a and a second opening 26b over the drain electrode 14 and the capacitor electrode 17, respectively. The reflector 26 also has a second transmissive hole 26c corresponding to the first transmissive hole 20. The reflector 26 is made of a metal that reflects light well such as aluminum (Al). A second passivation layer 28 is formed on the reflector 26. The second passivation layer 28 has a first contact hole 28a exposing the drain electrode 14 and a second contact hole 28b exposing the capacitor electrode 17 through the first passivation layer 18. The first contact hole 28a and the second contact hole 28b go through the first opening 26a and the second opening 26b, respectively. The second passivation layer 28 also has a third transmissive hole 28c corresponding to the first and second transmissive holes 20 and 26c. A transparent electrode 30 is formed on the second passivation layer 28. The transparent electrode 30 is located in the pixel region “P”. The transparent electrode 30 is connected to the drain electrode 14 through the first contact hole 28a and connected to the capacitor electrode 17 through the second contact hole 28b. Here, the reflector 26 is disconnected from the transparent electrode 30 so no electric charges are created in the reflector 26.
When voltage is applied to the transparent electrode 30, liquid crystal molecules (not shown), which are situated on the transparent electrode 30, are arranged and light incident from sunlight or artificial light is reflected at the reflector 26 according to the arrangement of the liquid crystal molecules. Thus, images are displayed in a reflective mode. On the other hand, when voltage is applied to the transparent electrode 30, light incident from a back light (not shown), which may be placed below the substrate 1, penetrates the transparent electrode 30 and the liquid crystal (not shown) through the second transmissive hole 26c depending on the alignment of the liquid crystal molecules. Therefore, light is emitted and images are displayed in a transmissive mode.
By the way, in the transflective liquid crystal display (LCD) device, amorphous silicon is generally used as the active layer 8 of the thin film transistor “T” because it can be uniformly formed at a low temperature over a large area. However, the amorphous silicon is sensitive to visible light. That is, when light is absorbed into the active layer 8 of the thin film transistor “T”, a leakage current caused by absorbed light flows in the thin film transistor “T”. This leakage current causes an undesirable signal in the LCD device, so that the thin film transistor “T” cannot properly function as a switching element. Therefore, a black matrix (not shown), which shields the thin film transistor “T” from the light, is formed on a substrate (not shown) opposing the substrate 1 having the thin film transistor “T” facing the thin film transistor “T”. However, it is difficult to completely shield the light with the black matrix because accurate arrangement of the black matrix and the thin film transistor “T” is not easy. If light is entirely shielded, the black matrix must have a larger size than the thin film transistor “T” if alignment margin is taken into consideration. Therefore, aperture ratio of the LCD device is reduced.